High power pulsed fiber lasers are currently in demand for a number of applications and uses. For example, numerous material processing applications such as memory repair, milling, micro-fabrication, drilling, etc. require pulsed laser systems which provide, among others, the four following characteristics all at the same time and with a great stability under different operating conditions and over time:                High pulse energy (50 μJ or higher) with excellent pulse amplitude stability, for processing material at the laser operating wavelength or for efficient frequency conversion;        Excellent beam quality (M2<1.1, astigmatism<10%, beam roundness>95%) with robust single mode operation, for superior processing quality, high throughput processes and efficient frequency conversion;        Narrow linewidth (Δλ<0.5 nm), for small spot sizes and efficient frequency conversion; and        Great flexibility in terms of control of the pulse temporal profile, such as pulse to pulse control over the temporal profile at high (>100 kHz) repetition rates.        
In other applications such as remote sensing of different chemical species, the source must additionally provide some level of flexibility over the pulse spectrum.
The advantages of laser materials processing with picosecond laser pulses are increasingly gaining attention in the industry. The time scale involved in such processes combines the benefits of light-matter interaction dynamics at both femtosecond and nanosecond regimes. For instance, laser light intensity on a work surface may be increased above the cold ablation threshold (even for high band gap materials), provided that the light pulses have sufficient energy for a given pulse duration (e.g. 10-100 μJ for a pulse duration of a few tens of picoseconds). A limited heat-affected zone on the processed material and little or no collateral damage is typically obtained from cold ablation performed with femtosecond optical pulses. However, femtosecond lasers are often complex and expensive. In addition, the ablation process is inherently slow, since the layer which is removed is usually very thin compared to that obtained using thermal ablation with nanosecond pulses. Finally, cold ablation may results in catastrophic damages when machining brittle materials.
Trains of picosecond laser pulses emitted at high repetition rates (>100 MHz) in a burst regime combine the benefits of both cold ablation and thermal ablation processes (see for example P. Forrester et al., “Effects of heat transfer and energy absorption in the ablation of biological tissues by pulse train-burst (>100 MHz) ultrafast laser processing”, Proc. of SPIE Vol. 6343, 63430J (2006); A. Nebel, et al., “Generation of tailored picosecond-pulse-trains for micro-machining”, Proc. of SPIE Vol. 6108, 6108-37 (2006); and U.S. Pat. No. 6,552,301, issued Apr. 22, 2003 to HERMAN et al.). Under such conditions, the time interval between successive pulses is short enough for heat to accumulate at the work surface, thus conditioning the material for subsequent ablation by multiphoton ionization with high laser beam intensities. Burst duration and pulse repetition rate provide unique control over the fluence delivery at the target. In turn, the latter is strongly tied to the processing speed and process quality. In effect, the pulse train burst characteristics allow the physical processes that depend on ultrashort laser pulses to be addressed separately from the characteristics of longer-time heat diffusion. This ensures clean ablation with smooth features.
Suitable bursts of picosecond pulses are generally obtained through a mode-locked is laser producing a picosecond pulse train combined with a slicer or pulse picker, which selects the pulses which constitute the “burst”. Actively mode-locked fiber lasers allow for the generation of picosecond pulses at high repetition rates, such as for example shown in U.S. Pat. No. 6,108,465 (LIDA et al.) and U.S. patent application published under no. 2006/0245456 A1(LASRI et al.). However, the timing between successive pulses cannot be adjusted arbitrarily; it is rather determined by the harmonics of the laser cavity and the fundamental pulse repetition frequency. Additionally, adjustable pulse durations of a few tens of picoseconds are difficult to obtain, since complex pulse shaping mechanisms occur along the pulse propagation within the fiber laser cavity. Semiconductor laser diodes may also be used for generation of picosecond pulses, either through active/passive mode-locking or through gain switching. However, both techniques suffer from serious drawbacks. For instance, pulse repetition rates below 10 GHz are hardly possible with electrically-pumped, mode-locked semiconductor laser diodes unless the device is inserted in an external cavity, while pulse shape and duration are barely adjustable in gain-switched semiconductor laser diodes. In either case, the energy of the emitted pulses is rarely above a few picoJoules.
Although other schemes are known for generating trains of picosecond pulses with relatively high repetition rates, they are not adapted to the generation of pulse train bursts, nor do they provide the flexibility and adaptability often useful for micromachining applications or the like. For example, U.S. Pat. Nos. 5,432,631 and 6,072,615, both to MAMYSHEV, teach of externally phase-modulated signals from cw lasers generating picosecond pulse trains for use in high data rate transmitters in optical fiber communications. See also P. V. Mamyshev et al., “Dual-wavelength source of high-repetition-rate, transform-limited optical pulses for soliton transmission”, Opt. Lett. 19(24), pp. 2074-2076 (1994), and E. A. Golovchenko et al., “Analysis of optical pulse train generation through filtering of an externally phase-modulated signal from a CW laser”, Electron. Lett. 31(16), pp. 1364-1366 (1995). The schemes discussed in these references fall short of energy (or fluence) since they do not address the specifics of pulsed fiber laser systems developed and used for light-matter interaction processes (e.g. material processing). In Z. Jiang, et al., “Optical processing based on spectral line-by-line pulse shaping on a phase-modulated CW laser”, IEEE J. Quantum Electron. 42(7), pp. 657-666 (2006), a similar approach is disclosed in the context of optical processing, but the line-by-line pulse shaper therein involves free space light propagation as well as the use of liquid crystal modulator, resulting in a system which may be too complex or costly for typical micromachining applications.
Solid-state gain media may also be used for high repetition rate ultrashort (e.g. picosecond) pulse lasers (see for example U.S. Pat. No. 6,778,565 (SPUEHLER et al.) U.S. Pat. No. 6,856,640 (HENRICH et al.)). Despite some prior art regarding tailoring of pulse train sequences emitted from such systems (see U.S. patent application published under no 2006/0018349 (KOPF et al.)), most schemes relying on solid-state lasers suffer from the same drawbacks as the methods mentioned above, i.e. fine tuning of both pulse repetition rate and pulse duration is virtually impossible. In addition, solid-state lasers lack the near diffraction-limited beam quality that sets apart fiber lasers and amplifiers from other types of laser sources.
There remains a need in the art for reliable, efficient and versatile tailoring methods and systems specifically dedicated to the generation of pulse train bursts of picosecond optical pulses, particularly for industrial applications purposes in material micro-machining.